This may not be the greatest idea, but try removing the shocks out of the rear and see how much more you could flex if you replaced them with longer ones? You may have to relocate the shock tabs, but it may be worth an inspection...
As for your IFS... I'd keep rocking that and save up for a SAS kit and a welder.
I have heard of people using the fj springs to lift their trucks or eliminate the ass-sag.. not sure if that would do the job but I could always try
